The present invention relates to controlled atmosphere storage, and more particularly to a system and method for improved scrubbing in controlled atmosphere storage.
Perishable items, such as post-harvest fruits and vegetables, are often stored in a gastight storage facilities. The atmosphere in even a gastight storage space will vary over time as a result of various factors. One of the more important factors affecting atmosphere is fruit respiration. Fruit respiration can have a significant affect on the oxygen and carbon dioxide levels in a storage room. Fruit respiration is the natural ripening process that occurs in fruits and vegetable after harvest. Respiration is the uptake of oxygen and the discharge of carbon dioxide just like the human body would do while breathing. The post-harvest life of fruits and vegetable life can be prolonged for an extended marketing period when placed in gastight refrigerated storage rooms. Normal respiration would reduce the oxygen in the space and also elevate the carbon dioxide.
To provide improved storage, post-harvest commodities are often stored in controlled atmosphere rooms (“CA rooms”) where factors, such as temperature and atmospheric composition, can be controlled to extend the life of the items. CA rooms typically include systems for monitoring and controlling temperature and atmospheric conditions (e.g. oxygen, carbon dioxide and nitrogen levels) in a gastight space. The atmospheric control systems often operate by repeatedly sampling gas levels within the CA room and adding or removing gases to maintain the atmosphere at one or more desired setpoints.
Because normal fruit respiration results in a reduction in oxygen and an increase in carbon dioxide, many CA rooms include carbon dioxide scrubber systems capable of removing carbon dioxide from the atmosphere of the CA room. A variety of different types of scrubber systems are currently in use and available from a number of well-known suppliers, such as Storage Control Systems, Inc. of Sparta, Mich. A typically carbon dioxide scrubber includes a scrubbing media that removes carbon dioxide from gas. For example, many conventional scrubbers use one or more beds of activated carbon as the scrubbing media. The activated carbon adsorbs carbon dioxide from CA room air that is blown through beds of activated carbon. The adsorption process slowly saturates the activated carbon with carbon dioxide. Once the activated carbon becomes sufficiently saturated, the activated carbon bed is regenerated by flushing the bed with low carbon dioxide air, such as ambient air.
Because the regeneration process takes time, scrubber systems have been developed that include two alternative scrubbing beds, such as two activated carbon beds. While one bed is used for scrubbing, the other bed can be regenerated. As a result, the overall system is capable of providing essentially continuous scrubbing.
Generally, ambient air is used to regenerate the activated carbon beds. Although ambient air is low in carbon dioxide, it is high in oxygen content relative to the CA room air. Accordingly, once a bed of this type is regenerated, it will typically contain high oxygen air. In typical systems of this type, air within the CA room is circulated through the scrubber and returned to the CA room in a closed loop. As a result, if no accommodation is made, the high oxygen air of the regenerated bed will be introduced into the CA room. The high oxygen air can raise the oxygen level in the CA room atmosphere to an undesirable level where it adversely impacts the function of the CA room.
To address this problem, some systems have been developed to flush the high oxygen air from the regenerated activated carbon scrubber beds with nitrogen before the regenerated activated carbon bed is used to scrub CA room air. In these systems, ambient air is used to regenerate the activated carbon bed. Once the bed is sufficiently regenerated, nitrogen is introduced into the bed to reduce the oxygen content. Typically, a predetermined amount of nitrogen will be introduced into each bed following regeneration and prior to its next scrubbing cycle. Although a nitrogen flush system helps to resolve the concerns associated with high oxygen air, there is a significant cost associated with the generation of nitrogen used to flush the regenerated scrubber beds.
As an alternative to nitrogen flush systems, some systems have been developed to use the low oxygen air present in an activated carbon scrubber bed to help address the problem of high oxygen air in regenerated scrubber beds. For example, some existing systems extract low oxygen air from an activated carbon scrubber bed before introducing ambient air into the bed for regeneration. The air is typically extracted by closing off the circulation loop and pumping air from the scrubber bed back into the room, which temporarily pressurizes the CA room. The extracted air is introduced into a regenerated bed to drive off some of the higher oxygen air, thereby returning the pressure in the CA room back to normal and reducing the overall oxygen content of the air in the regenerated bed. Although the return of extracted low oxygen air helps to reduce the oxygen in the regenerated bed, it is typically not sufficient to lower the oxygen content to the desired level.